A printer of the type involved herein is shown in applicant's joint U.S. Pat. No. 3,952,153, issued Apr. 20, 1976. With printers of this type the iron of the transformer has so far always been magnetized in the same direction. In order to prevent the operating point of the magnetic field of the transformer from being too much in the upper part of the B-H-curve, the iron core can be provided with an air gap. In order to obtain the then required relatively strong magnetizing current, so that a secondary current of the order of 10 A can be supplied during 5 ms, many turns are needed and the transformer has to be rather large. As the printer according to the above mentioned patent with seven printing elements, called rulers or bars, requires seven transformers, weight and dimensions play a very important part, particularly in the case of a supply apparatus for a cheap subscriber's set of small dimensions.